The present invention relates to semiconductor manufacturing equipment. More particularly, the present invention relates to a system and method for delivering pulsed radio frequency (RF) power to a plasma processing chamber. A plasma processing chamber may be used for dry etching, or plasma-assisted deposition, or other processes.
One embodiment of the present invention relates to a system and method for delivering pulsed RF power to a dry etching chamber. Dry etching basically involves exposing a semiconductor wafer to an electrical discharge and a reactive gas at the same time. Dry etching includes plasma etching, reactive ion etching, and reactive sputtering.
In conventional dry etching, continuous wave (CW) RF power is delivered to the plasma chamber. One problem with using CW RF power for plasma processing is that charge-up damage may occur.
Delivering pulsed power prevents charge from building up and so mitigates charge-up damage effects. Prior art systems for delivering pulsed power include the ones described in U.S. Pat. No. 4,863,549 (Grunwald), U.S. Pat. No. 4,891,118 (Ooiwa et al.), U.S. Pat. No. 5,087,857 (Ahn), and U.S. Pat. No. 5,362,358 (Yamagata et al.). However, the prior art pulsed power systems suffer from various defects and disadvantages.
First, the prior art systems do not address the complex impedance mismatch problem which exists due to the pulsing of the power. Minimizing the amount of power that is reflected back towards the power generator from the processing chamber becomes more difficult for pulsed power. If the reflected power is too high, the generator may be detrimentally driven into foldback.
Second, the prior art systems do not address the complexities and difficulties in accurately measuring power delivered by the pulsed signal. Measuring the amount of energy delivered is needed so that manufacturing processes may be repeated in a highly predictable way.
The delivery process for pulsed RF power is more complex than the delivery process for CW RF power. In the CW case, the load impedance of the plasma chamber evolves in stages including an initial ignition stage when the plasma is ignited, a transient stage as the plasma approaches some steady-state condition, and a steady-state stage at which the plasma maintains the steady-state condition. Conventional CW RF power delivery systems have been designed to drive power into the steady-state plasma at a high level of stability, while merely tolerating the ignition and transient stages. These conventional systems operate on control algorithms with iteration times varying from 100 microseconds to a few milliseconds. Such time scales largely define the limits of temporal variation in load impedances that the conventional systems may compensate so as to deliver a highly stable level of power.
In the stable steady-state condition, the CW power transfer into the chamber is defined by only one impedance value and two voltage levels, where the impedance value corresponds to the impedance of the transmission line connecting the generator and chamber, and the two voltages corresponds to the amplitude of the forward and reflected voltage waves traveling on the transmission line. Reflected power is minimized by fixing degrees of freedom in the local matching circuit such that the transmission line impedance is matched during the steady-state condition, and the power transfer may be characterized with only the steady state impedance and two voltage amplitude measurements (forward and reflected amplitudes).
In contrast, in the pulsed case, the load impedance of the plasma chamber has no steady-state behavior. This lack of steady-state behavior significantly increases the complexity of the situation. The load impedance is repeatedly cycling from a value near ignition to a terminal value which can be held for as short a time as a few microseconds. Hence, the power delivery system must deal with transient conditions, instead of steady-state conditions. Conventional CW power delivery systems are not equipped to deal with such transient conditions.
In particular, the load impedance varies in time, so more is needed than fixing the local match""s degrees of freedom in the local matching circuit. In order to deal with the cycling of the local impedance, determinations must be made on which degrees of freedom are available to be varied, how to control the degrees of freedom to accomplish such variation, and how to vary the degrees of freedom to minimize reflected power. In addition, the voltage amplitudes of the forward and reflected power waves vary in time, so simple measurements of voltage amplitudes do not yield an accurate measure of the power transmitted.
The present invention provides a system and method for overcoming the above-described problems relating to the delivery of pulsed RF power to a plasma processing chamber. The power reflected from the chamber is reduced using one or more of the following techniques: (1) varying the RF frequency within a pulse period; (2) ramping up the pulse heights at the leading edge of the pulse train; (3) simultaneously transmitting a relatively low CW signal along with the pulsed signal; and (4) rapidly switching the shunt capacitance within a local matching network within a pulse period. The amount of power delivered to the plasma by the pulses is measured by way of a time-averaging mechanism coupled to a directional coupler connected to the transmission line. The time-averaging mechanism may comprise circuitry to measure temperatures of loads attached to the directional coupler, or analog integrating circuitry attached to the directional coupler, or digital integrating circuitry attached to the directional coupler.
In accordance with a first aspect of the present invention, a method for delivering RF power to a plasma comprises: generating a RF oscillation having a frequency and an amplitude; modulating the frequency of the RF oscillation; modulating the amplitude of the RF oscillation; amplifying the RF oscillation; and transmitting the RF oscillation to the plasma. In one embodiment of this aspect, a method for delivering pulsed RF power to a plasma comprises: generating a pulse train comprising a series of pulses, each pulse being characterized by a pulse period; generating a frequency-varied RF oscillation which varies in frequency within the pulse period; modulating the frequency-varied RF oscillation with the pulse train to generate a frequency-varied pulsed RF signal; and transmitting the frequency-varied pulsed RF signal to the plasma.
In accordance with a second aspect of the present invention, a method for delivering pulsed radio frequency (RF) power to a plasma comprises: generating a RF oscillation; generating a pulse train comprising a series of pulses, each pulse being characterized by a pulse height; varying the pulse height from pulse to pulse in the pulse train to form a pulse train having varied pulse heights; modulating the RF oscillation with the pulse train having varied pulse heights to form a pulsed RF signal having varied pulse heights; and transmitting the pulsed RF signal having varied pulse heights to the plasma.
In accordance with a third aspect of the present invention, a method for delivering pulsed radio frequency (RF) power to a plasma comprises: generating a first continuous wave RF signal and a second continuous wave RF signal; generating a pulse train comprising a series of pulses; modulating the first continuous wave RF signal with the pulse train to form a pulsed RF signal; and transmitting simultaneously both the pulsed RF signal and the second continuous wave RF signal to the plasma.
In accordance with a fourth aspect of the present invention, a method for delivering pulsed RF power to a plasma comprises: transmitting a pulsed RF signal by way of an impedance matching network to the plasma, the pulsed RF signal including a series of pulses, each pulse being characterized by a pulse period; sensing a magnitude error signal related to a magnitude of a reflected signal coming back from the plasma; and utilizing the magnitude error signal for control over a variable shunt capacitor in the impedance matching network, where capacitance of the variable shunt capacitor is varied within the pulse period.
Finally, in accordance with a fifth aspect of the present invention, a system for delivering pulsed radio frequency (RF) power to a plasma and for measuring power absorbed by the plasma comprises: a pulsed RF power generator configured to generate forward RF pulses; a RF cable having a first end coupled to the pulsed RF power generator, the RF cable being adapted to carry the forward RF pulses; a reaction chamber coupled to a second end of the RF cable, the reaction chamber being adapted to contain the plasma and to couple the forward RF pulses to the plasma; a directional coupler coupled to the RF cable, the directional coupler being adapted to generate a first attenuated signal proportional to the forward RF pulses and a second attenuated signal proportional to the backward RF pulses; and a time-averaging mechanism coupled to the directional coupler and adapted to time-average the first attenuated signals to generate a first time-averaged signal and to time-average the second attenuated signal to generate a second time-averaged signal, where the power absorbed by the plasma is proportional to the difference between the first and second time averaged signals.